8.6 NOTES Types of Reactions Double Replacement

2. Double replacement reactions These occur if one of 3 products is formed: a gas, a precipitate or a molecular compound such as water. See more detail in section b.

a. Reactions in Aqueous Solutions i. Aqueous solutions - some ionic compounds dissociate (break into ions) in water 3-D network of ions is destroyed - ions are separated and surrounded by water molecules (known as hydration) - other ionic compounds have low solubility in water and will NOT dissolve; use a solubility chart to determine if forms a precipitate.; only S will be aqueous

Solubility Rule #1 – Li, Na, K, Rb, Cs (group I metals), NH4+, C2H3O2-, NO3 -, ClO3 -, ClO4 – are soluble.

ii. Reactions that form precipitates These all involve ionic species which exist as ions in solution. Examples: Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)   3NaOH(aq) + Al(NO3)3(aq)  Al(OH)3(s) + 3NaNO3

iii. Reactions that form water All involve an acid and a base. Examples: HCl + NaOH  H2O + NaCl   H2SO4 + 2NaOH  Na2SO4 + 2H2O

iv. Reactions that form gases CO2, HCN, and H2S are the most common gases formed. Examples: 2HI(aq) + Li2S(aq)  H2S(g) + 2LiI(aq)   2HCl(aq) + Na2CO3(aq)  H2CO3(aq) + 2NaCl(aq) H2CO3(aq)  H2O(l) + CO2(g)

b. Ionic Equations i. Molecular equation Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq) ii. Complete ionic equation to show rxn accurately, better to write ionic species as ions; Pb+2(aq) + 2NO3-(aq) + 2K+(aq) + 2I- (aq)  PbI2(s) +2K+(aq) + 2NO3-(aq) iii. Spectator ions ions that are not involved in the overall rxn K+ and NO3-; appear on both sides of the reaction and are unchanged; iv. Net ionic equation shows only the species that actually take part in the reaction Pb+2(aq) + 2I-(aq)  PbI2(s)